1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus including the scanning optical apparatus, which are suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, and a multifunction printer which use an electrophotographic process.
2. Description of the Related Art
Conventionally, in a scanning optical apparatus of the laser beam printer (LBP) or the digital copying machine, a light beam emitted from a light source unit after being modulated in accordance with an image signal is deflected periodically by a deflecting element (optical deflector) constituted of a polygon mirror. Then, the deflected light beam is condensed like a spot onto a recording medium surface of a photosensitive drum or the like by an imaging optical system having an fθ characteristic so as to scan the surface for recording an image.
There is known a color image forming apparatus including a tandem type scanning optical apparatus that includes multiple scanning and imaging functions described above, and optically scans the surfaces to be scanned corresponding to different hues simultaneously by different light beams (Japanese Patent Application Laid-Open Nos. 2002-196269 and 2005-084565). The tandem type scanning optical apparatus used for the color image forming apparatus has multiple light source units, and multiple light beams from the multiple light source units enter one or more deflecting elements. In this case, the multiple light beams enter the deflecting element at angles with respect to each of a deflection surface and a surface that includes a rotation axis of the deflecting element and is orthogonal to the deflection surface. Then, after passing through one or more imaging lenses, the light beams are reflected and separated by a mirror or the like, so that multiple light spots of the light beams simultaneously scan multiple different surfaces to be scanned, respectively.
In this tandem type scanning optical apparatus, when one of scanning optical systems scans using the light beam, the light beam reflected by the lens surface of the imaging lens may enter another scanning optical system on the opposing side with respect to the deflecting element and may reach a surface to be scanned to be light of ghost or light of flare. Japanese Patent Application Laid-Open No. 2002-196269 discloses a case where a tandem type scanning optical apparatus is provided with a light shielding member that shields light of ghost entering a vicinity of the deflecting element from the scanning optical system in the opposing direction. Japanese Patent Application Laid-Open No. 2005-084565 discloses a case where a tandem type scanning optical apparatus is provided with a mirror having a low reflectance (light amount reducing unit) disposed between a scanning optical element closest to the surface to be scanned and the deflecting element, so as to reduce light of ghost generated from the scanning optical element closest to the surface to be scanned.
In the method of reducing light of ghost (light of flare) using the light shielding member as described in Japanese Patent Application Laid-Open No. 2002-196269, the light shielding member disposed in the vicinity of the deflecting element is close to the deflecting element itself or an effective light beam. For this reason, it is difficult to shield the light of ghost completely. In the method of reducing light of ghost using the light amount reducing unit as described in Japanese Patent Application Laid-Open No. 2005-084565, it is possible to reduce light of ghost generated from the scanning optical element closest to the surface to be scanned. However, light of ghost generated from an optical element closer to the deflecting element than the mirror having a low reflectance cannot be reduced.
In the tandem type scanning optical apparatus in which multiple scanning optical systems are disposed to be opposed with respect to the deflecting element, it is important for forming an accurate image to prevent a light beam entering from the opposing side with respect to the deflecting element from becoming light of flare or light of ghost with a simple method. In particular, as for a color image forming apparatus, even if light of ghost slightly enters a photosensitive surface corresponding to an image of a hue having low brightness, for example, a black image, the light of ghost can be visual, with the result that image quality is deteriorated.